In image recording on a full-color heat-sensitive recording material, a thermal head directly contacts the surface of the recording material for thermal recording. Thus, if the surface of the recording material is rough, an uneven image quality will result. It is known that flatness of a recording material greatly depends on flatness of a support.
Conventionally, material used advantageously as a support is polyethylene-coated paper, which is manufactured by coating base paper from pulp with polyethylene kneaded with a white pigment or the like. However, use of polyethylene-coated paper as a support results in rough rippled gloss due to the rough base paper surface on which a polyethylene layer is formed, and thus uniformity of picture quality is considerably degraded.
In order to solve the above-described drawback, there have been proposed methods which do not use base paper as a support, but only use a thermoplastic resin film as a support. A polystyrene-based resin film which contains a white pigment as a filler is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 49-114921 and Japanese Patent Application Publication (JP-B) No. 55-5104. According to UK Patent Nos. 1,563,591 and 1,563,592, barium sulfate is added to polyester, and the resultant mixture is spread. Japanese Patent Application Publication (JP-B) No. 56-4901 discloses a technique wherein barium sulfate and titanium dioxide are added to a thermoplastic resin. Japanese Patent Application Laid-Open (JP-A) No. 61-118746 discloses a technique wherein surface-treated titanium dioxide having an average grain size of 0.1 to 0.5 .mu.m is added to polyester. These disclosed supports are hard and fragile, and thus are likely to generate crack when handled. Also, they give a stiff feel like plastic and a strange textural feel as compared with a support composed of base paper and a polyolefin resin layer melt-extruded on the base paper.
According to Japanese Patent Application Laid-Open (JP-A) No. 49-12815, a polyolefin layer which contains a white inorganic filler is formed on one side of synthetic paper or on one side of plastic film which enables writing thereon. However, after the thus-formed support is covered with a recording layer, the resultant recording material does not allow adjustment of a curl.
Also, Japanese Patent Application Laid-Open (JP-A) No. 3-91740 discloses a support wherein paper, synthetic paper, or a film substrate is coated with a polyolefin resin compound which contains a certain type of titanium dioxide. However, the thus-formed support fails to provide sufficient smoothness and readiness of handling.
Japanese Patent Application Laid-Open (JP-A) No. 2-230236 discloses a technique wherein a film of a thermoplastic resin such as a vinyl chloride resin, which is used as material for ID cards and cash cards because of good embossing property, is laminated with a polyolefin resin. However, the thus-formed support gives a stiff feel like a plastic sheet and a strange textural feel and fails to provide sufficient smoothness, as compared with a support composed of base paper and a polyolefin resin layer melt-extruded on the base paper.